Acoustic emission, to which the present invention particularly applies, is concerned with the detection of elastic waves that are emitted from a source within an object and become manifest at positions remote from the source. Acoustic emission signals are also emitted from various points of an object spontaneously at various times. Often acoustic emission occurs as a result of growth of cracks. Such cracks may arise from the application of various kinds of forces to the object such as mechanical pressure or because of temperature changes or even chemical action.
By way of example, vessels employed in refineries emit acoustic signals often when they are subjected to increasing pressure. Rocket motors, or at least prototypes thereof, are often tested in advance of use by subjecting them to increasing pressure thereby generating acoustic emission signals. Acoustic emission may also arise spontaneously from unidentified causes while an object is standing apparently undisturbed.
The location of cracks and other sources of acoustic emission has been determined for many years by locating acoustic emission detectors at various points on the surface of the object under investigation, detecting the signals received and the relative times of arrival thereof at the respective detectors, and then either manually or automatically calculating the origin of the acoustic emission signals by triangulation or similar methods, taking into account the relative times of arrival of waves at the detectors and the propogation speed of the waves. The severity of the phenomenon, such as cracking, that accounts for the acoustic emission, has been determined by observing the strength of the waves arriving at the detectors such as by counting the number of pulses or groups of pulses detected in a given interval of time or over an extended time.
Many other signals that occur simultaneously constitute noise and therefore, if also detected, may obscure the acoustic emission phenomenon under investigation. Various methods have been employed for improving the signal-to-noise ratio. This invention is concerned with a new type of acoustic emission transducer for selectively detecting acoustic emission signals in a predetermined high frequency band in preference to other signals.